The present invention relates to a printed wiring board and manufacture thereof and, more particularly, to an improved printed wiring board and an improved manufacturing method thereof in which conductive layers are formed into predetermined circuit patterns on both the upper and lower sides of at least the insulating layer, and a conducting section is formed in a portion of the insulating layer so as to enable electrical connection between the upper and lower conductive layers.
An a result of the debut of digital copiers and color copiers, high-frequency signals are now used. With this situation, countermeasures against EMI (Electromagnetic Interference) according to the VCCI regulations (Voluntary Control Council Interference) pose technically important problem to the business machine industry.
For example, a wiring component used for the transmission of communication and image signals within a digital copier in required to simultaneously satisfy demands for an electromagnetic shielding capability and for the capability of stably transmitting high-frequency signals. In many cases, a shielded electric wire is usually employed an the wiring component. However, the electric wire itself in expensive, and it is difficult to assemble an electric wire for a power source and electric wires for a high-frequency signal into a single wiring component, which in turn prevents reduction in the size of the product.
Printed wiring boards provided with the same noise leak prevention function as that of the shielded electric wire are proposed as means for solving the previously-described problem. For example, a double-sided flexible printed wiring board (hereinafter referred to as a double-sided FPC as required) is formed into a microstrip line structure. One of two sides of the double-sided FPC in used as a signal layer, and the other side is used an a shielding layer (a ground layer). Further, there in proposed a two-layer single-sided flexible printed wiring board (hereinafter referred to an a two-layer single-sided PPC) [the Examined Japanese Patent Application Publication No. Hei 6-48753]. This FPC is produced by forming a signal layer and a shielding layer on respective insulating bases, and by stacking the thus-formed signal layer and the shielding layer into a two-layer structure. The two-layer single-sided flexible printed wiring board is provided with a strip line structure.
The shielding layers of the microstrip line structure and the strip line structure shield the signal layer and suppress the leakage of noise by distorting high-frequency signals to such an extent as to provide no practical problems.
According to a known method of manufacturing the double-sided FPC, an insulating resin base (insulating base), such as polyimide, having both sides coated with metal foil, such as iron, aluminum, or copper, is used. Holes layer are formed in the insulating resin base in order to enable electrical connection between conductor circuits provided on both sides of the insulating resin. The insulating resin base is entirely plated, and conductive circuits (such as a signal layer and a shielding layer) are formed by a known method such as etching.
In contrast, according to another known method of manufacturing the two-layer single-sided FPC, two single-sided metal-foiled plates are formed by coating with iron, aluminum, or copper one side of each of two insulating resin bases (insulating bases) made of polyimide or polyester. Conductor circuits (such an a signal layer and a shielding layer) are formed on the respective FPCs by a known method such as etching. Holes are formed in the desired locations of an insulating resin layer of the respective single-sided metal-foiled plates. Subsequently, the single-sided metal-foiled plates are laminated, and electrical connection between the signal layer and the shielding layer is established by applying conductive paste to the holes.
There often arises the case of handling both a digital signal and an analog signal in one printed wiring board.
However, the required electric characteristics of the printed wiring board change according to whether the printed wiring board handles a digital signal or an analog signal. It is very difficult to handle the signals mixedly.
More specifically, it is necessary to round the waveform of a digital signal to a desired extent while suppressing ringing or reflection. In contrast, it is necessary to faithfully transmit an analog signal without rounding or distorting its waveform and to protect the analog signal from external noise.
At this time, the electric characteristics of the printed wiring board are determined by the thickness and dielectric constant of an insulating layer and by the width and geometry of a wiring pattern. If the electric characteristics are determined according to either the analog or digital signal, the other signal, for example, the analog signal, may be rounded or distorted too much to be practically used. Conversely, in the case of the digital signal, the leakage of noise such an electromagnetic waves increases because of ringing or reflection.
As described above, it in difficult to control the electric characteristics of the printed wiring board so as to satisfy both signals. Particularly, in a case where the frequency of the signal is speeded up, this phenomenon becomes noticeable.
To solve the previously described technical problems, it in conceivable that a printed wiring board for handling a digital signal in independent of a printed wiring board for handling an analog signal, and the electric characteristics of each of the printed wiring boards are controlled so an to match with the corresponding signal. This method adds to the cost of components and therefore is impractical.
Another means for solving the problems in a conductor circuit (a conductor circuit which require small electric capacitance) for handling, e.g., an analog signal, in which only an conductive layer of a shielding layer provided on a circuit pattern possessing small electric capacitance is formed into a mesh or a lattice. As a result, the analog signal becomes more susceptible to external electromagnetic-wave problems, and there arisen another technical problem such as unstable characteristic impedance of the conductor circuit.